The Dynamical Transition of Lipid Multilamellar Bilayers as a Matter of Cooperativity

The present study is the application of a two-state model formerly developed by Bicout and Zaccai [ Bicout, D. J. and Zaccai, G. Biophys. J. 2001, 80 (3), 1115-1123 ] to describe the dynamical transition exhibited in the atomic mean square displacements of biological samples in terms of dynamic and thermodynamic parameters. Data were obtained by elastic incoherent neutron scattering on 1,2-dimyristoyl-sn-glycero-3-phosphocholine lipid membranes in various hydration states and on one partially per-deuterated lipid membrane. Fitting the data with the model allowed investigating which parts of lipid molecules were mainly involved in the dynamical transition, heads, tails, or both. Clear differences were found between the fully protonated and partially deuterated membranes. These findings shed light on the question of what is the degree of dynamical cooperativity of the atoms during the transition. Whereas the level of hydration does not significantly affect it, as the dry, the intermediate dry, and fully hydrated membranes all undergo a rather broad transition, the transition of the lipid tails is much sharper and sets in at much lower temperature than that of the heads. Therefore, the dynamical cooperativity appears high among the particles in the tails. Moreover, the transition of the lipid tails has to be completed first before the one of the head groups starts.